Retracting mechanism of a zoom lens barrel

ABSTRACT

A retracting mechanism of a zoom lens barrel includes a first through third lens group moving rings; a first linear guide projection formed on the first lens group moving ring to be engaged in a linear guide slot from outside the second lens group moving ring, the first linear guide projection including a hanging groove formed therealong, a rear end of the hanging groove being closed; a second linear guide projection formed on the third lens group moving ring to be engaged in the linear guide slot from inside the second lens group moving ring; and a linear moving key projecting from a front end of the second linear guide projection to be engaged in the hanging groove. A rear moving limit of the third lens group moving ring is determined by contact of the linear moving key with the closed end of the hanging groove.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a retracting mechanism of a zoomlens barrel.

[0003] 2. Description of the Related Art

[0004] A zoom lens system including a first lens group, a second lensgroup and a third lens group in that order from the object side, whereinthe first lens group and the third lens group are integrally moved alongan optical axis during a variation of a focal length, is known in theart. A retractable zoom lens barrel including such a type of zoom lenssystem, wherein a integral movement relationship between the first lensgroup and the third lens group is canceled to bring the first lens groupclose to the second and third lens groups to reduce the length of thezoom lens barrel when it is retracted to a retracted position(full-retracted position or a power-off position), is also known in theart. In general, a compression coil spring for biasing the second lensgroup and the third lens group in opposite directions away from eachother is installed therebetween so that the third lens group is movedrearward to its rear moving limit relative to the first lens group bythe spring force of the compression coil spring to establish theaforementioned integral movement relationship between the first lensgroup and the third lens group when the focal length of the zoom lenssystem is varied (when the zoom lens barrel is in a ready-to-photographstate), and so that the third lens group is brought close to the secondlens group (and the first lens group) by compressing the compressioncoil spring when the zoom lens barrel is retracted to the retractedposition.

[0005] Such a conventional zoom lens barrel is usually provided with amoving ring which supports the third lens group and is guided along theoptical axis of the zoom lens system, and is further provided betweenthe moving ring and another element of the zoom lens barrel with alinear guide mechanism for guiding the moving ring linearly along theoptical axis. If an excessive load is applied to the moving ring, anengagement of the moving ring with the another element of the zoom lensbarrel through the linear guide mechanism is disengaged, which may causethe moving ring from coming off the zoom lens barrel.

[0006] A solution to this problem is to increase the strength of thelinear guide mechanism by forming the linear guide mechanism so as tohave a complicated structure. However, if the linear guide mechanism iscomplicated, it will be difficult to install the movable ring into thezoom lens barrel during assembly.

SUMMARY OF THE INVENTION

[0007] The present invention provides a retracting mechanism of a zoomlens barrel including the aforementioned type of zoom lens system, inwhich the first lens group and the third lens group are moved togetheras one body along an optical axis during a variation of a focal length,wherein the retracting mechanism can be easily assembled, and whereineach of the first, second and third lens groups can be linearly guidedwith reliability.

[0008] According to an aspect of the present invention, a retractingmechanism of a zoom lens barrel is provided, including a first lensgroup, a second lens group and a third lens group, in that order from anobject side, wherein the first lens group and the third lens group areintegrally moved along an optical axis thereof during a zoomingoperation, and wherein at least one of the first lens group and thethird lens group is moved relative to the other to reduce a distancetherebetween when the zoom lens barrel is retracted, the retractingmechanism including a first lens group moving ring which is linearlyguided along the optical axis, and supports the first lens group; asecond lens group moving ring which is linearly guided along the opticalaxis, and supports the second lens group; a third lens group moving ringwhich is linearly guided along the optical axis, and supports the thirdlens group, the third lens group moving ring being allowed to freelyapproach the first lens group moving ring while being prevented frommoving away from the first lens group moving ring beyond a moving limitrelative to the first lens group moving ring; a cam mechanism for movingthe first lens group moving ring and the second lens group moving ringin respective moving manners independent of each other along the opticalaxis; a biasing device for biasing the third lens group moving ring in adirection away from the first lens group moving ring; a linear guidethrough-slot formed on the second lens group moving ring to be elongatedin a direction parallel to the optical axis; a first linear guideprojection formed on the first lens group moving ring to be engaged inthe linear guide through-slot from outside the second lens group movingring, the first linear guide projection including a hanging grooveformed along a substantially center thereof and elongated in a directionparallel to the optical axis, a rear end of the hanging groove beingclosed; a second linear guide projection formed on the third lens groupmoving ring to be engaged in the linear guide through-slot from insidethe second lens group moving ring; and a linear moving key projectingfrom a front end of the second linear guide projection to be engaged inthe hanging groove. A rear moving limit of the third lens group movingring relative to the first lens group moving ring is determined bycontact of the linear moving key with the closed rear end of the hanginggroove.

[0009] It is desirable for the hanging groove to include a narrow-widthgroove portion which communicatively connects with the linear guidethrough-slot; and a wide-width groove portion which communicativeconnects with the narrow-width groove portion, a width of the wide-widthgroove portion in a circumferential direction of the first lens groupmoving ring being greater than that of the narrow-width groove portion.It is desirable for the linear moving key to include a neck portionwhich is engaged in the narrow-width groove portion; and a head portionwhich is engaged in the wide-width groove portion, a width of the headportion in a circumferential direction of the first lens group movingring being greater than that of the neck portion.

[0010] It is desirable for the second lens group moving ring to includea follower introducing through-slot which extends orthogonal to thelinear guide through-slot to communicatively connect with the linearguide through-slot; and a first follower introducing groove whichextends parallel to the optical axis to communicative connect with thefollower introducing through-slot, a front end of the first followerintroducing groove communicatively connecting with the followerintroducing through-slot, a rear end of the first follower introducinggroove being open on a rear surface of the second lens group movingring. The first lens group moving ring includes a second followerintroducing groove which radially communicatively connects with thefollower introducing through-slot and the hanging groove when the firstlens group moving ring is positioned at a specific position relative tothe second lens group moving ring in the optical axis direction. Thelinear moving key is inserted into the hanging groove via the followerintroducing through-slot, the first follower introducing groove and thesecond follower introducing groove during assembly of the zoom lensbarrel.

[0011] It is desirable for the first lens group moving ring, the secondlens group moving ring and the third lens group moving ring to becoaxially arranged so that the first lens group moving ring ispositioned around the second lens group moving ring, and so that thesecond lens group moving ring is positioned around the third lens groupmoving ring.

[0012] It is desirable for the cam mechanism to include a cam ring whichis positioned around the second lens group moving ring to be rotatablerelative to the second lens group moving ring, and includes a pluralityof outer cam grooves formed on an outer peripheral surface of the camring, and a plurality of inner cam grooves formed on an inner peripheralsurface of the cam ring; a plurality of inward cam followers whichproject radially inwards from the first lens group moving ring to beengaged in the plurality of outer cam grooves, respectively; and aplurality of outward cam followers which project radially outwards fromthe second lens group moving ring to be engaged in the plurality ofinner cam grooves, respectively.

[0013] It is desirable for the biasing device to be a compression coilspring.

[0014] It is desirable for positions of the first lens group moving ringand the second lens group moving ring in the optical axis direction tobe adjustable by rotating the cam ring to make the second followerintroducing groove and the follower introducing through-slot aligned inthe optical axis direction so that the second follower introducinggrooves, the follower introducing through-slots and the first followerintroducing grooves form an L-shaped follower introducing channel,through which the linear moving key is inserted into the hanging groove,when the third lens group moving ring is installed in the zoom lensbarrel during assembly.

[0015] The present disclosure relates to subject matter contained inJapanese Patent Application No. 2003-034959 (filed on Feb. 13, 2003)which is expressly incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be described below in detail withreference to the accompanying drawings in which:

[0017]FIG. 1 is a diagram showing reference moving paths of zoom lensgroups of a zoom lens system provided in an embodiment of a zoom lensbarrel according to the present invention;

[0018]FIG. 2 is an exploded perspective view in axial section of thezoom lens groups and lens support frames therefor;

[0019]FIG. 3 is a longitudinal cross sectional view of the embodiment ofthe zoom lens barrel according to the present invention, showing anupper half of the zoom lens barrel from the optical axis thereof in aretracted state;

[0020]FIG. 4 is a view similar to that of FIG. 3, and shows an upperhalf of the zoom lens barrel from the optical axis thereof at thewide-angle extremity;

[0021]FIG. 5 is a view similar to that of FIG. 3, and shows a lower halfof the zoom lens barrel from the optical axis thereof at the telephotoextremity;

[0022]FIG. 6 is a transverse cross sectional view taken along VI-VI lineshown in FIG. 3;

[0023]FIG. 7 is a transverse cross sectional view taken along VII-VIIline shown in FIG. 3;

[0024]FIG. 8 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3;

[0025]FIG. 9 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3;

[0026]FIG. 10 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3, showing a first lens group moving ring andperipheral elements;

[0027]FIG. 11 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3, showing a third lens group moving ring andperipheral elements;

[0028]FIG. 12 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3, showing a second lens group moving ring andperipheral elements;

[0029]FIG. 13 is a longitudinal view of a portion of the zoom lensbarrel shown in FIG. 3, showing a portion of the second lens groupmoving ring and peripheral elements;

[0030]FIG. 14 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3, showing a stationary barrel, a pulse motorsupported by the stationary barrel, and peripheral elements, seen fromthe rear side thereof;

[0031]FIG. 15 is an exploded perspective view of a portion of the zoomlens barrel shown in FIG. 3, showing the stationary barrel, a fourthlens group and peripheral elements;

[0032]FIG. 16 is a developed view of a cam/helicoid ring, showing a setof first cam grooves of the cam/helicoid ring for moving the first lensgroup and a set of third cam grooves of the cam/helicoid ring for movingan exterior ring;

[0033]FIG. 17 is a developed view of the first lens group moving ring,the second lens group moving ring and the third lens group moving ring,showing linear guide mechanical linkages among the first through thirdlens group moving rings;

[0034]FIG. 18 is an enlarged view of a portion of the developed viewshown in FIG. 17; and

[0035]FIG. 19 is a developed view of the cam/helicoid ring, showing theshapes of a set of second cam grooves of the cam/helicoid ring formoving the second lens group.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0036] First of all, a zoom lens system (zoom lens optical system)provided in an embodiment of a zoom lens barrel of a camera according tothe present invention will be hereinafter discussed with reference toFIGS. 1 through 5. The zoom lens system of the zoom lens barrel 10 is avari-focal lens system consisting of four lens groups: a positive firstlens group L1, a negative second lens group L2, a positive third lensgroup L3 and a positive fourth lens group L4, in that order from theobject side (left side as viewed in FIG. 3). The first through thirdlens groups L1, L2 and L3 are moved relative to one another along anoptical axis O to vary the focal length of the zoom lens system and thefourth lens group L4 is moved along the optical axis O to make a slightfocus adjustment, i.e., to adjust a slight focus deviation caused by thevariation of the focal length. During the operation of varying the focallength of the zoom lens system between wide angle and telephoto, thefirst lens group L1 and the third lens group L3 move along the opticalaxis while maintaining the distance therebetween. The fourth lens groupL4 also serves as a focusing lens group. FIG. 1 shows both moving pathsof the first through fourth lens groups L1 through L4 during the zoomingoperation and moving paths for advancing/retracting operation. Bydefinition, a vari-focal lens is one whose focal point slightly varieswhen varying the focal length, and a zoom lens is one whose focal pointdoes not vary substantially when varying the focal length. However, thevari-focal lens system of the present invention is also hereinafterreferred to as a zoom lens system.

[0037] The overall structure of the zoom lens barrel 10 will behereinafter discussed with reference to FIGS. 1 through 19. The zoomlens barrel 10 is provided with a stationary barrel 11 which is fixed toa camera body (not shown). As shown in FIG. 8, the stationary barrel 11is provided on an inner peripheral surface thereof with a femalehelicoid 11 a and a set of three linear guide grooves 11 b which extendparallel to the optical axis O. The zoom lens barrel 10 is providedinside the stationary barrel 11 with a cam/helicoid ring (cam ring) 12.As shown in FIG. 9, the cam/helicoid ring 12 is provided, on an outerperipheral surface thereof in the vicinity of the rear end of thecam/helicoid ring 12, with a male helicoid 12 a which is engaged withthe female helicoid 11 a of the stationary barrel 11. The cam/helicoidring 12 is provided on the thread of the male helicoid 12 a with a spurgear 12 b which is always engaged with a drive pinion 13 (see FIG. 15).The drive pinion 13 is provided in a recessed portion 11 c (see FIG. 3)formed on an inner peripheral surface of the stationary barrel 11. Thedrive pinion 13 is supported by the stationary barrel 11 to be freelyrotatable in the recessed portion 11 c on an axis of the drive pinion13. Accordingly, forward and reverse rotations of the drive pinion 13cause the cam/helicoid ring 12 to move forward rearward along theoptical axis O while rotating about the optical axis O due to theengagement of the drive pinion 13 with the spur gear 12 b and theengagement of the female helicoid 11 a with the male helicoid 12 a. Inthe present embodiment of the zoom lens barrel 10, the cam/helicoid ring12 is the only element thereof which rotates about the optical axis O.

[0038] The zoom lens barrel 10 is provided around the cam/helicoid ring12 with a linear guide ring 14. The linear guide ring 14 is provided, onan outer peripheral surface thereof at the rear end of the linear guidering 14, with a set of three linear guide projections 14 a which projectradially outwards to be engaged in the set of three linear guide grooves11 b of the stationary barrel 11, respectively. The linear guide ring 14is provided, on an inner peripheral surface thereof at the rear end ofthe linear guide ring 14, with a set of three bayonet lugs 14 b (onlyone of them appears in FIGS. 1 through 4). The cam/helicoid ring 12 isprovided, on an outer peripheral surface thereof immediately in front ofthe male helicoid 12 a (the spur gear 12 b), with a circumferentialgroove 12 c in which the set of three bayonet lugs 14 b are engaged tobe rotatable about the optical axis O in the circumferential groove 12c. Accordingly, the linear guide ring 14 is linearly movable along theoptical axis O together with the cam/helicoid ring 12 without rotatingabout the optical axis O.

[0039] The zoom lens barrel 10 is provided around the cam/helicoid ring12 with a first lens group moving ring (first lens frame) 15 whichsupports the first lens group L1, and is further provided around thefirst lens group moving ring 15 with an exterior ring 16 serving as alight shield member. The zoom lens barrel 10 is provided inside thecam/helicoid ring 12 with a second lens group moving ring (second lensframe) 17 which supports the second lens group L2. As shown in FIGS. 4,9 and 16, the cam/helicoid ring 12 is provided on an outer peripheralsurface thereof with a set of three first cam grooves C15 for moving thefirst lens group moving ring 15 and a set of three third cam grooves C16for moving the exterior ring 16, and is further provided on an innerperipheral surface of the cam/helicoid ring 12 with a set of six secondcam grooves C17 for moving the second lens group moving ring 17 (seeFIG. 19). The set of three first cam grooves C15 and the set of threethird cam grooves C16 are slightly different in shape, and are apartfrom one another at predetermined intervals in a circumferentialdirection of the cam/helicoid ring 12. The set of six second cam groovesC17 have the same basic cam diagrams, and includes three front secondcam grooves C17, and three rear second cam grooves C17 which arepositioned behind the three front second cam grooves C17 in the opticalaxis direction (vertical direction as viewed in FIG. 19), respectively;the three front second cam grooves C17 are apart from one another in acircumferential direction of the cam/helicoid ring 12 while the threerear second cam grooves C17 are apart from one another in acircumferential direction of the cam/helicoid ring 12. Each of the firstlens group moving ring 15, the exterior ring 16 and the second lensgroup moving ring 17 is linearly guided along the optical axis O. Arotation of the cam/helicoid ring 12 causes the first lens group movingring 15, the exterior ring 16 and the second lens group moving ring 17to move along the optical axis O in accordance with the contours of theset of three first cam grooves C15, the set of three third cam groovesC16 and the set of six second cam grooves C17, respectively.

[0040] Linear guide mechanical linkages among the first lens groupmoving ring 15, the exterior ring 16 and the second lens group movingring 17 will be discussed hereinafter. As shown in FIGS. 4 and 5, thefirst lens group moving ring 15 is provided with an outer ring portion15X, an inner ring portion 15Y and a flange wall 15Z by which the frontend of the outer ring portion 15X and the front end of the inner ringportion 15Y are connected to have a substantially U-shaped crosssection. The cam/helicoid ring 12 is positioned between the outer ringportion 15X and the inner ring portion 15Y. Three cam followers 15 awhich are respectively engaged in the set of three first cam grooves C15are fixed to the outer ring portion 15X in the vicinity of the rear endthereof. The zoom lens barrel 10 is provided with a first lens groupsupport frame 24 which supports the first lens group L1. As shown inFIGS. 8 and 9, the first lens group support frame 24 is fixed to theinner ring portion 15Y at the front end thereof through a male threadportion and a female thread portion which are formed on an outerperipheral surface of the first lens group support frame 24 and an innerperipheral surface of the inner ring portion 15Y, respectively (see FIG.10). The first lens group support frame 24 can be rotated relative tothe first lens group moving ring 15 to adjust the position of the firstlens group support frame 24 along the optical axis O relative to thefirst lens group moving ring 15 to carry out a zooming adjustment (whichis an adjustment operation which is carried out in a manufacturingprocess of the zoom lens barrel if necessary).

[0041] The linear guide ring 14, which is linearly guided along theoptical axis O by the stationary barrel 11, is provided, on an innerperipheral surface thereof at approximately equi-angular intervals(intervals of approximately 120 degrees), with a set of three linearguide grooves 14 c (only one of them appears in FIG. 9), while the outerring portion 15X of the first lens group moving ring 15 is provided atthe rear end thereof with a set of three linear guide projections 15 b(see FIG. 10) which project radially outwards to be engaged in the setof three linear guide grooves 14 c, respectively. The outer ring portion15X is provided with a set of three assembly slots 15 c (see FIGS. 10and 16), and is further provided at the rear ends of the set of threeassembly slots 15 c with a set of linear guide slots 15 d which arecommunicatively connected with the set of three assembly slots 15 c andare smaller in width than the set of three assembly slots 15 c,respectively. Three linear guide keys 16 a which are fixed to theexterior ring 16 which is positioned between the outer ring portion 15Xand the linear guide ring 14 are engaged in the set of linear guideslots 15 d, respectively. The maximum relative moving distance betweenthe first lens group moving ring 15 and the exterior ring 16 along theoptical axis O (the difference in shape between the set of three firstcam grooves C15 and the set of three third cam grooves C16) is only aslight distance, and the length of each linear guide slot 15 d in theoptical axis direction is correspondingly short. A set of three camfollowers 16 b which are engaged in the set of three third cam groovesC16 are fixed to the set of three linear guide keys 16 a, respectively(see FIGS. 7 and 9).

[0042] The zoom lens barrel 10 is provided between the first lens groupmoving ring 15 and the exterior ring 16 with a compression coil spring19 (see FIGS. 3 through 5). The compression coil spring 19 biases thefirst lens group moving ring 15 rearward to remove backlash between theset of three first cam grooves C15 and the set of three cam followers 15a, and at the same time, biases the exterior ring 16 forward to removebacklash between the set of three third cam grooves C16 and the set ofthree cam followers 16 b.

[0043] As shown in FIG. 16, the set of three first cam grooves C15 andthe set of three third cam grooves C16 are shaped slightly differentfrom each other in their respective retracting positions, as comparedwith their respective photographing ranges (zooming ranges), so that theexterior ring 16 advances from the photographing position thereofrelative to the first lens group moving ring 15 to prevent barrierblades of a lens barrier unit 30 (see FIG. 8) and the first lens groupL1 from interfering with each other when the zoom lens barrel 10 isfully retracted as shown in FIG. 3. More specifically, as shown in FIG.16, the shapes of the first cam grooves C15 and the third cam groovesC16 are determined so that the distance Q in the optical axis directionbetween the first cam grooves C15 and the third cam grooves C16 in thepreparation ranges (i.e., the range between the retracted position andthe position at which the lens barrier unit 30 is fully open) is longerthan that of the zoom ranges (i.e., the range between the wide-angleextremity and the telephoto extremity). Namely, throughout the entiretyof the preparation ranges the distance Q=Q1, however, the distance Qgradually reduces from a position OP2 at a predetermined distance from afully opened position OP1 of the lens barrier unit 30 (i.e., from aposition whereby the first lens group L1 and the lens barrier unit 30 donot interfere with each other), so that the distance Q=Q2 (<Q1) at thewide-angle extremity, and the distance Q=Q2 in the entirety of the zoomranges.

[0044] It can be seen in FIG. 3 that a clearance c1 between the flangewall 15Z of the first lens group moving ring 15 and a flange wall 16 fof the exterior ring 16 when the zoom lens barrel 10 is in the retractedposition is greater than that when the zoom lens barrel 10 is in aready-to-photograph position as shown in FIG. 4 or 5. In other words,when the zoom lens barrel 10 is in a ready-to-photograph position asshown in FIG. 4 or 5, the flange wall 15Z of the first lens group movingring 15 and the flange wall 16 f of the exterior ring 16 are positionedclosely to each other to reduce the length of the zoom lens barrel 10.The lens barrier unit 30 is supported by the exterior ring 16 at thefront end thereof. The zoom lens barrel 10 is provided, immediatelybehind the lens barrier unit 30 (between the lens barrier unit 30 andthe flange wall 16 f of the exterior ring 16), with a barrieropening/closing ring 31 (see FIG. 9). Rotating the barrieropening/closing ring 31 at the retracted position via rotation of thecam/helicoid ring 12 causes the barrier blades of the lens barrier unit30 to open and shut. The mechanism for opening and closing the barrierblades using a barrier opening/closing ring such as the barrieropening/closing ring 31 is known in the art. Note that in theillustrated embodiment, although the shapes of the first cam grooves C15and the third cam grooves C16 are determined so that the distance Q(i.e., Q2) is constant (unchanging) over the entire zoom range, thedistance Q (i.e., Q2) can be determined so as to change in accordancewith the focal length. Furthermore, the distance Q2 over the zoom rangecan be determined so as to be greater than the distance Q1 over thepreparation range.

[0045] The front end of each third cam groove C16 is open on a front endsurface of the cam/helicoid ring 12 to be formed as an open end C16 a(see FIG. 16) through which the associated cam follower 16 b of theexterior ring 16 is inserted into the third cam groove C16. Likewise,the front end of each first cam groove C15 is open on a front endsurface of the cam/helicoid ring 12 to be formed as an open end C15 a(see FIG. 16) through which the associated cam follower 15 a of thefirst lens group moving ring 15 is inserted into the first cam grooveC15.

[0046] The inner ring portion 15Y of the first lens group moving ring 15is provided on an inner peripheral surface thereof with a set of threelinear guide projections 15 f which are elongated in a directionparallel to the optical axis O, while the second lens group moving ring17 is provided with a set of three linear guide slots (linear guidethrough-slots) 17 a which are elongated in a direction parallel to theoptical axis O to be engaged with the set of three linear guideprojections 15 f to be freely slidable relative thereto along theoptical axis O (see FIGS. 6, 7 and 17). Each linear guide projection 15f is provided along a substantially center thereof with a hanging groove15 e which is elongated in a direction parallel to the optical axis Oand which has a substantially T-shaped cross section as shown in FIG. 6.The three linear guide projections 15 f and the three linear guide slots17 a constitute a first linear guide mechanism. The rear end of eachhanging groove 15 e is closed (see FIGS. 17 and 18). The second lensgroup moving ring 17 is provided on an outer peripheral surface thereofwith six cam followers 17 c which are engaged in the set of six secondcam grooves C17 of the cam/helicoid ring 12, respectively.

[0047] The zoom lens barrel 10 is provided inside the second lens groupmoving ring 17 with a third lens group moving ring (third lens frame) 18which supports the third lens group L3. The third lens group moving ring18 is provided on an outer peripheral surface thereof with a set ofthree linear guide projections 18 a which are elongated in a directionparallel to the optical axis O to be engaged in the set of three linearguide slots 17 a of the second lens group moving ring 17 to be freelyslidable relative thereto along the optical axis O, respectively. Thethird lens group moving ring 18 is provided on a center of each linearguide projection 18 a at the front end thereof with a linear moving key(stop projection) 18 b (see FIGS. 11, 17 and 18) which has asubstantially T-shaped cross section to be engaged in the associatedhanging groove 15 e. The three linear guide projections 15 f, the threehanging groove 15 e and the three linear moving keys 18 b constitute asecond linear guide mechanism. Furthermore, the three linear guide slots17 a and the three linear guide projections 18 a constitute a thirdlinear guide mechanism. As shown in FIG. 11, the zoom lens barrel 10 isprovided with a shutter unit 20 which is inserted into the third lensgroup moving ring 18 to be positioned in front of the third lens groupL3. The shutter unit 20 is fixed to the third lens group moving ring 18by a fixing ring 20 a. The zoom lens barrel 10 is provided between thethird lens group moving ring 18 (the fixing ring 20 a) and the secondlens group moving ring 17 with a compression coil spring 21 whichcontinuously biases the third lens group moving ring 18 rearwardsrelative to the second lens group moving ring 17. The rear limit of thisrearward movement of the third lens group moving ring 18 relative to thesecond lens group moving ring 17 is determined by the three linearmoving keys 18 b contacting the closed rear ends of the three hanginggrooves 15 e, respectively. Namely, when the zoom lens barrel 10 is in aready-to-photograph position, each linear moving key 18 b remains incontact with the rear end of the associated hanging groove 15 e of thefirst lens group moving ring 15 to keep the distance between the firstlens group L1 and the third lens group L3 constant. When the zoom lensbarrel 10 changes from a ready-to-photograph state to the retractedstate shown in FIG. 3, a further rearward movement of the first lensgroup L1 in accordance with contours of the set of three first camgrooves C15, after the third lens group L3 (the third lens group movingring 18) has reached the mechanical rear moving limit thereof, causesthe first lens group L1 to approach the third lens group L3 whilecompressing the compression coil spring 21 (see FIG. 1). Each linearmoving key 18 b is formed so that the radially outer end thereof bulgesto be prevented from coming off the associated hanging groove 15 e.

[0048] Although a biasing force of the compression coil spring 21 can beapplied directly to the second lens group moving ring 17 (i.e., althoughthe second lens group L2 can be fixed to the second lens group movingring 17), the second lens group L2 is made to be capable of movingrearward relative to the second lens group moving ring 17 for thepurpose of further reduction in length of the zoom lens barrel 10 in theretracted state thereof in the present embodiment of the zoom lensbarrel. FIGS. 12 and 13 show this structure for the further reduction inlength of the zoom lens barrel 10. The second lens group moving ring 17is provided at the front end thereof with a cylindrical portion 17 ehaving an inner flange 17 d. Three linear guide grooves 17 f, whichextend parallel to the optical axis direction and open at the front andrear ends thereof, are formed at equi-angular intervals on thecylindrical portion 17 e. The zoom lens barrel 10 is provided inside thesecond lens group moving ring 17 with an intermediate ring (intermediatemember) 25. The intermediate ring 25 is provided at the front endthereof with a flange portion 25 a which is fitted in the cylindricalportion 17 e to be freely slidable on the cylindrical portion 17 e inthe optical axis direction. An end portion of the compression coilspring 21 abuts against the flange portion 25 a, so that the flangeportion 25 a presses against the inner flange 17 d due to the resiliencyof the compression coil spring 21. Three guide projections 25 d whichradially extend outwards are provided on the outer peripheral surface ofthe flange portion 25 a. The three guide projection 25 d arerespectively engaged with the three linear guide grooves 17 f of thesecond lens group moving ring 17 from the rear side of the second lensgroup moving ring 17. Accordingly, the intermediate ring 25 is preventedfrom rotating about the optical axis with respect to the second lensgroup moving ring 17, and can only relatively move in the optical axisdirection. The front face of the flange portion 25 a can move forwardsuntil sliding contact is made with the rear face of the inner flange 17d. The zoom lens barrel L2 is provided inside the second lens groupmoving ring 17 with a second lens group support frame 26 to which thesecond lens group L2 is fixed. A male thread 26 b of the second lensgroup support frame 26 is screwed into female thread 25 e formed on theinner periphery of the intermediate ring 25. Accordingly, the positionof the second lens group L2 relative to the intermediate ring 25 whichis prevented from rotating about the optical axis can be adjusted in theoptical axis direction (zooming adjustment) by rotating the second lensgroup support frame 26 relative to the intermediate ring 25. After thisadjustment, the second lens group support frame 26 can be permanentlyfixed to the intermediate ring 25 by putting drops of an adhesive agentinto a radial through hole 25 b formed on the intermediate ring 25. Thesecond lens group support frame 26 is provided on an outer peripheralsurface thereof with an outer flange 26 a, and a clearance C2 (see FIG.13) for the zooming adjustment exits between a front end surface of theinner flange 17 d and the outer flange 26 a. The compression coil spring21 biases the intermediate ring 25 forward, and the intermediate ring 25is held at a position where the flange portion 25 a contacts with theinner flange 17 d when the zoom lens barrel 10 is in aready-to-photograph state. Namely, on the one hand, the position of thesecond lens group L2 is controlled by the set of six second cam groovesC17 when the zoom lens barrel 10 is in a ready-to-photograph state; onthe other hand, the second lens group support frame 26 is pushedrearward mechanically by the rear end of the first lens group supportframe 24 to thereby move the outer flange 26 a of the second lens groupsupport frame 26 rearward to a point where the outer flange 26 acontacts with the inner flange 17 d when the zoom lens barrel 10 isretracted to the retracted position thereof. This reduces the length ofthe zoom lens barrel 10 by a length corresponding to the clearance C2.

[0049] The zoom lens barrel 10 is provided immediately behind theintermediate ring 25 with a light shield ring 27 which is supported bythe intermediate ring 25. As shown in FIG. 12, the light shield ring 27is provided with a ring portion 27 a and a set of three leg portions 27b which extend forward from the ring portion 27 a at intervals ofapproximately 120 degrees. Each leg portion 27 b is provided at thefront end thereof with a hook portion 27 c which is formed by bendingthe tip of the leg portion 27 b radially outwards. The intermediate ring25 is provided on an outer peripheral surface thereof with a set ofthree engaging holes 25 c with which the hook portions 27 c of the setof three leg portions 27 b are engaged, respectively (see FIG. 12). Thezoom lens barrel 10 is provided between the light shield ring 27 and thesecond lens group support frame 26 with a compression coil spring 28having a substantially truncated conical shape which continuously biasesthe light shield ring 27 rearwards. When the zoom lens barrel 10 isretracted toward the retracted position, the light shield ring 27approaches the second lens group support frame 26 while compressing thecompression coil spring 28 after reaching the rear moving limit of thelight shield ring 27. The lengths of the set of three engaging holes 25c in the optical axis direction are determined to allow the ring portion27 a to come into contact with the second lens group support frame 26.

[0050] The compression coil spring 28 also serves as a device forremoving backlash between the intermediate ring 25 and the second lensgroup support frame 26 when the second lens group support frame 26 isrotated relative to the intermediate ring 25 for the aforementionedzooming adjustment. The zooming adjustment is performed by rotating thesecond lens group support frame 26 relative to the intermediate ring 25to adjust the position of the second lens group L2 in the optical axisdirection relative to the intermediate ring 25 while viewing theposition of an object image. This zooming adjustment can be performedwith precision with backlash between the intermediate ring 25 and thesecond lens group support frame 26 being removed by the compression coilspring 28.

[0051] The zoom lens barrel 10 is provided behind the third lens groupmoving ring 18 with a fourth lens group support frame 22 to which thefourth lens group L4 is fixed. As described above, the fourth lens groupL4 is moved to make a slight focus adjustment to the vari-focal lenssystem to adjust a slight focal deviation thereof while the firstthrough third lens groups L1, L2 and L3 are moved relative to oneanother to vary the focal length of the zoom lens system, and is alsomoved as a focusing lens group. The fourth lens group L4 is moved alongthe optical axis O by rotation of a pulse motor 23 (see FIGS. 5 and 14).The pulse motor 23 is provided with a rotary screw shaft 23 a. A nutmember 23 b is screwed on the rotary screw shaft 23 a to be preventedfrom rotating relative to the stationary barrel 11. The nut member 23 bis continuously biased by an extension coil spring S in a direction tocontact with a leg portion 22 a which projects radially outwards fromthe fourth lens group support frame 22 (see FIGS. 5 and 15). The fourthlens group support frame 22 is prevented from rotating by guide bars 22b, which extend in direction parallel to the optical axis direction,which are slidably engaged with radial projecting followers 22 c whichextend radially outwards from the fourth lens group support frame 22(see FIGS. 2 and 15). Accordingly, rotations of the pulse motor 23forward and reverse cause the fourth lens group support frame 22 (thefourth lens group L4) to move forward and rearward along the opticalaxis O, respectively. Rotations of the pulse motor 23 are controlled inaccordance with information on focal length and/or information on objectdistance.

[0052] Accordingly, in the above described embodiment of the zoom lensbarrel, rotating the cam/helicoid ring 12 by rotation of the drivepinion 13 causes the first lens group moving ring 15, the exterior ring16 and the second lens group moving ring 17 to move along the opticalaxis O in accordance with contours of the set of three first cam groovesC15, the set of three third cam grooves C16 and the set of six secondcam grooves C17, respectively. When the first lens group moving ring 15moves forward from the retracted position, firstly the three linearmoving keys 18 b contact the rear ends of the three hanging grooves 15e, respectively, and subsequently the third lens group moving ring 18moves together with the first lens group moving ring 15 with the threelinear moving key 18 b remaining in contact with the rear ends of thethree hanging grooves 15 e, respectively. The position of the fourthlens group L4 is controlled by the pulse motor 23, whose rotations arecontrolled in accordance with information on focal length, to make aslight focus adjustment to the vari-focal lens system to adjust a slightfocal deviation thereof. As a result, reference moving paths as shown inFIG. 1 for performing a zooming operation are obtained. Rotations of thepulse motor 23 are also controlled in accordance with information onobject distance to perform a focusing operation.

[0053] As described above, the present embodiment of the zoom lensbarrel includes the first lens group L1, the second lens group L2, thethird lens group L3 and the fourth lens group L4, in that order from theobject side. The first through third lens groups L1 through L3 are movedalong the optical axis O to vary the focal length of the zoom lenssystem. During this variation of focal length, the first lens group L1and the third lens group L3 are integrally moved (i.e., with a constantdistance therebewteen) along the optical axis O. In addition, the threecam followers 15 a of the first lens group moving ring 15, which ispositioned around the cam/helicoid ring 12 and supports the first lensgroup L1, are respectively engaged in the set of three first cam groovesC15 of the cam/helicoid ring 12, while the six cam followers 17 c of thesecond lens group moving ring 17, which is positioned inside thecam/helicoid ring 12 and supports the second lens group L2, arerespectively engaged in the set of six second cam grooves C17 of thecam/helicoid ring 12. The three cam followers 15 a, the three first camgrooves C15, the six cam followers 17 c and the six second cam groovesC17 are elements of the cam mechanism of the zoom lens barrel 10 towhich the present invention is applied.

[0054] In addition, the first lens group moving ring 15 is linearlyguided along the optical axis O by the engagement of the set of threelinear guide projections 15 b, which project radially outwards from theouter ring portion 15X of the first lens group moving ring 15, with theset of three linear guide grooves 14 c of the linear guide ring 14,which is linearly guided along the optical axis O by the stationarybarrel 11.

[0055] The second lens group moving ring 17 is linearly guided along theoptical axis O by the engagement of the set of three linear guideprojections 15 f, which project radially inwards from the inner ringportion 15Y of the first lens group moving ring 15, with the set ofthree linear guide slots 17 a of the second lens group moving ring 17.

[0056] The third lens group moving ring 18 is linearly guided along theoptical axis O by the second lens group moving ring 17; specifically, bythe engagement of the set of three linear guide projections 18 a of thethird lens group moving ring 18 with the set of three linear guide slots17 a of the second lens group moving ring 17. Additionally, the thirdlens group moving ring 18 is linearly guided along the optical axis O bythe first lens group moving ring 15 by the engagement of the set ofthree linear moving keys (stop projections) 18 b, each of which projectsradially outwards from the front end of the associated linear guideprojection 18 a, with the set of three hanging grooves 15 e, each ofwhich is formed on the associated linear guide projection 15 f. Namely,opposite edges of each linear guide projection 15 f of the inner ringportion 15Y guide the second lens group moving ring 17 linearly alongthe optical axis O via the set of three linear guide slots 17 a, while acentral portion of each linear guide projection 15 f (i.e., each hanginggroove 15 e) guides the third lens group moving ring 18 linearly alongthe optical axis O. This structure miniaturizes the linear guidemechanism for guiding the second lens group moving ring 17 and the thirdlens group moving ring 18 by effectively using three peripheral surfacesof each linear guide projection 15 f. In addition, each linear guideslot 17 a is formed to be slidably fitted on opposite side edges of theassociated linear guide projection 15 f and opposite side edges of theassociated linear guide projections 18 a so that the radial thickness ofeach linear guide projection 15 f and the radial thickness of theassociated linear guide projection 18 a are substantially accommodatedwithin the wall thickness of the second lens group moving ring 17. Thisstructure makes it possible to increase the strength of the zoom lensbarrel so that each of the second lens group moving ring 17 and thethird lens group moving ring 18 can be reliably guided linearly alongthe optical axis O without requiring an increase in diameter of the zoomlens barrel 10. Moreover, the zoom lens barrel 10 has been miniaturizedto be smaller than a conventional similar zoom lens barrel due to theabove described structure wherein the outer ring portion 15X of thefirst lens group moving ring 15 is linearly guided along the opticalaxis O by the linear guide ring 14 while each of the second lens groupmoving ring 17 and the third lens group moving ring 18 is linearlyguided along the optical axis O by the inner ring portion 15Y of thefirst lens group moving ring 15. Accordingly, since the third lens groupmoving ring 18 is linearly guided along the optical axis O by twomembers: the first lens group moving ring 15 and the second lens groupmoving ring 17, the third lens group moving ring 18 can be linearlyguided with stability even if an excessive load is applied to either thefirst lens group moving ring 15 or the second lens group moving ring 17,while the third lens group moving ring 18 is prevented from coming offthe zoom lens barrel 10 even if an excessive load is applied to thethird lens group moving ring 18.

[0057] In addition, as shown in FIG. 6, each hanging groove 15 e isprovided with a narrow-width groove portion 15 e 1 and a wide-widthgroove portion 15 e 2 whose width in a circumferential direction of thefirst lens group moving ring 15 is greater than that of the narrow-widthgroove portion 15 e 1. Each linear moving key 18 b is provided with aneck portion 18 b 1 which is engaged in the associated narrow-widthgroove portion 15 e 1, and a head portion 18 b 2 which bulges to have awidth greater than the width of the neck portion 18 b 1 in thecircumferential direction of the first lens group moving ring 15 andwhich is fixed at the radially outer end of the neck portion 18 b 1 tobe engaged in the wide-width groove portion 15 e 2. Since the headportion 18 b 2 of each linear moving key 18 b is not only engaged in thewide-width groove portion 15 e 2 of the associated hanging groove 15 e,to prevent the neck portion 18 b 1 from being disengaged from thenarrow-width groove portion 15 e 1 of the associated hanging groove 15e, but also is linearly guided by the wide-width groove portion 15 e 2of the associated hanging groove 15 e, the third lens group moving ring18 is linearly guided along the optical axis O with stability via thefirst lens group moving ring 15.

[0058] The rear end of each hanging groove 15 e is closed, and thedistance between the first lens group moving ring 15 and the third lensgroup moving ring 18 becomes maximum when the three linear moving keys18 b contact with the closed rear ends of the three hanging grooves 15e, respectively. In addition, the compression coil spring (biasingdevice) 21, which biases the second lens group moving ring 17 (the firstlens group moving ring 15) and the third lens group moving ring 18 inopposite directions away from each other, is installed in a compressedfashion between the intermediate ring 25 and the third lens group movingring 18.

[0059] Additionally, in the above illustrated embodiment of the zoomlens barrel, the third lens group moving ring 18 is linearly guidedalong the optical axis O by the first lens group moving ring 15 and thesecond lens group moving ring 17, and also each linear moving key 18 band the associated hanging groove 15 e are formed to have special(corresponding) shapes, making it easy for the zoom lens barrel 10 to beassembled.

[0060] As shown in FIGS. 17 through 19, the second lens group movingring 17 is provided with a set of three follower introducingthrough-slots 17 a 1 which extend orthogonal to the set of three linearguide slots 17 a to communicatively connect with the set of three linearguide slots 17 a, respectively. The second lens group moving ring 17 isprovided on an inner peripheral surface thereof with a set of threefollower introducing grooves (bottomed grooves) 17 a 2 which extendparallel to the optical axis O to communicate with the set of threefollower introducing through-slots 17 a 1. Specifically, the front endof each follower introducing groove 17 a 2 communicatively connectedwith the associated follower introducing through-slot 17 a 1, and therear end of each follower introducing groove 17 a 2 is open on a rearsurface of the second lens group moving ring 17. The inner ring portion15Y is provided on an inner peripheral surface thereof with a set ofthree follower introducing grooves (bottomed grooves) 15 al which areformed to be associated with the three hanging grooves 15 e,respectively. More specifically, each follower introducing groove 15 alis formed on an inner peripheral surface of the inner ring portion 15Yso as to extend through a side wall of the associated linear guideprojection 15 f (which extends parallel to the optical axis O) to extendbetween the hanging groove 15 e of the associated linear guideprojection 15 f and the outside thereof in a circumferential directionof the inner ring portion 15Y. The length of each follower introducinggrooves 15 al and the length of each follower introducing through-slot17 a 1 in the circumferential direction of the inner ring portion 15Yare greater than that of the head portion 18 b 2 of the associatedlinear moving key 18 b.

[0061] When the third lens group moving ring 18 is installed in the zoomlens barrel 10 during assembly, firstly the positions of the first lensgroup moving ring 15 and the second lens group moving ring 17 in theoptical axis direction are adjusted by rotating the cam/helicoid ring 12to make the three follower introducing grooves 15 al and the threefollower introducing through-slots 17 a 1 aligned in the optical axisdirection (see FIG. 17) so that the three follower introducing grooves15 a 1, the three follower introducing through-slots 17 a 1 and thethree follower introducing grooves 17 a 2 form three L-shaped followerintroducing channels, respectively. Subsequently, in a state where eachhead portion 18 b 2 of each linear moving key 18 b is aligned with eachassociated follower introducing groove 17 a 2 in the optical axisdirection, respectively, the third lens group moving ring 18 is insertedfrom its front end into the rear end of the second lens group movingring 17 so that each head portion 18 b 2 of each linear moving key 18 bis engaged in the associated follower introducing groove 17 a 2 of thelinear guide slot 17 a. Subsequently, if the third lens group movingring 18 is further inserted into the second lens group moving ring 17,each head portion 18 b 2 reaches the junction between the associatedfollower introducing groove 17 a 2 and the associated followerintroducing through-slot 17 a 1 through the associated followerintroducing groove 17 a 2. In this state where each head portion 18 b 2is positioned at the junction between the associated followerintroducing groove 17 a 2 and the associated follower introducingthrough-slot 17 a 1, a rotation of the third lens group moving ring 18relative to the second lens group moving ring 17 in a predeterminedrotational direction causes each linear moving key 18 b to rotate in thecircumferential direction of the inner ring portion 15Y to move into thehanging groove 15 e of the associated linear guide projection 15 f viathe associated follower introducing through-slot 17 a 1 and theassociated follower introducing groove 15 al so that the neck portion 18b 1 and the head portion 18 b 2 of the associated linear moving key 18 bare engaged in the narrow-width groove portion 15 e 1 and the wide-widthgroove portion 15 e 2 of the associated hanging groove 15 e,respectively. In this state, if the cam/helicoid ring 12 is rotated sothat the three follower introducing grooves 15 al and the three followerintroducing through-slots 17 a 1 are not aligned in the optical axisdirection, the set of three linear moving keys 18 b cannot rotate aboutthe optical axis O relative to the first lens group moving ring 15. Thiscompletes the installation of the third lens group moving ring 18. Itshould be noted that each linear moving key 18 b is not disengaged fromthe associated hanging groove 15 e through the associated followerintroducing through-slot 17 a 1 and the introducing groove 15 a 1 tothereby prevent the third lens group moving ring 18 from coming off thezoom lens barrel 10 when the zoom lens barrel 10 is in aready-to-photograph position or the retracted position because the threefollower introducing grooves 15 al and the three follower introducingthrough-slots 17 a 1 are not aligned in the optical axis direction.

[0062] The zoom lens barrel which has been discussed above withreference to FIGS. 1 through 19 is just an example to which a retractingmechanism devised according to the present invention is applied. Thepresent invention can be applied not only to a zoom lens barrel such asthe above described zoom lens barrel 10, but also to any other zoom lensincluding a cam ring and a lens support ring, regardless of whether thecam ring includes a helicoid such as the male helicoid 12 a of thecam/helicoid ring 12.

[0063] As can be understood from the foregoing, according to the presentinvention, a retracting mechanism of a zoom lens barrel including a zoomlens system in which the first lens group and the third lens group aremoved together as one body along an optical axis during a variation of afocal length, wherein the retracting mechanism can be easily assembledand further wherein each of the first, second and third lens groups canbe linearly guided with reliability, is achieved.

[0064] Obvious changes may be made in the specific embodiment of thepresent invention described herein, such modifications being within thespirit and scope of the invention claimed. It is indicated that allmatter contained herein is illustrative and does not limit the scope ofthe present invention.

What is claimed is:
 1. A retracting mechanism of a zoom lens barrelincluding a first lens group, a second lens group and a third lensgroup, in that order from an object side, wherein said first lens groupand said third lens group are integrally moved along an optical axisthereof during a zooming operation, and wherein at least one of saidfirst lens group and said third lens group is moved relative to theother to reduce a distance therebetween when said zoom lens barrel isretracted, said retracting mechanism comprising: a first lens groupmoving ring which is linearly guided along said optical axis, andsupports said first lens group; a second lens group moving ring which islinearly guided along said optical axis, and supports said second lensgroup; a third lens group moving ring which is linearly guided alongsaid optical axis, and supports said third lens group, said third lensgroup moving ring being allowed to freely approach said first lens groupmoving ring while being prevented from moving away from said first lensgroup moving ring beyond a moving limit relative to said first lensgroup moving ring; a cam mechanism for moving said first lens groupmoving ring and said second lens group moving ring in respective movingmanners independent of each other along said optical axis; a biasingdevice for biasing said third lens group moving ring in a direction awayfrom said first lens group moving ring; a linear guide through-slotformed on said second lens group moving ring to be elongated in adirection parallel to said optical axis; a first linear guide projectionformed on said first lens group moving ring to be engaged in said linearguide through-slot from outside said second lens group moving ring, saidfirst linear guide projection including a hanging groove formed along asubstantially center thereof and elongated in a direction parallel tosaid optical axis, a rear end of said hanging groove being closed; asecond linear guide projection formed on said third lens group movingring to be engaged in said linear guide through-slot from inside saidsecond lens group moving ring; and a linear moving key projecting from afront end of said second linear guide projection to be engaged in saidhanging groove, wherein a rear moving limit of said third lens groupmoving ring relative to said first lens group moving ring is determinedby contact of said linear moving key with said closed rear end of saidhanging groove.
 2. The retracting mechanism according to claim 1,wherein said hanging groove comprises: a narrow-width groove portionwhich communicatively connects with said linear guide through-slot; anda wide-width groove portion which communicative connects with saidnarrow-width groove portion, a width of said wide-width groove portionin a circumferential direction of said first lens group moving ringbeing greater than that of said narrow-width groove portion, whereinsaid linear moving key comprises: a neck portion which is engaged insaid narrow-width groove portion; and a head portion which is engaged insaid wide-width groove portion, a width of said head portion in acircumferential direction of said first lens group moving ring beinggreater than that of said neck portion.
 3. The retracting mechanismaccording to claim 1, wherein said second lens group moving ringcomprises: a follower introducing through-slot which extends orthogonalto said linear guide through-slot to communicatively connect with saidlinear through-guide slot; and a first follower introducing groove whichextends parallel to said optical axis to communicative connect with saidfollower introducing through-slot, a front end of said first followerintroducing groove communicatively connecting with said followerintroducing through-slot, a rear end of said first follower introducinggroove being open on a rear surface of said second lens group movingring, wherein said first lens group moving ring comprises a secondfollower introducing groove which radially communicatively connects withsaid follower introducing through-slot and said hanging groove when saidfirst lens group moving ring is positioned at a specific positionrelative to said second lens group moving ring in said optical axisdirection, and wherein said linear moving key is inserted into saidhanging groove via said follower introducing through-slot, said firstfollower introducing groove and said second follower introducing grooveduring assembly of said zoom lens barrel.
 4. The retracting mechanismaccording to claim 1, wherein said first lens group moving ring, saidsecond lens group moving ring and said third lens group moving ring arecoaxially arranged so that said first lens group moving ring ispositioned around said second lens group moving ring, and so that saidsecond lens group moving ring is positioned around said third lens groupmoving ring.
 5. The retracting mechanism according to claim 1, whereinsaid cam mechanism comprises: a cam ring which is positioned around saidsecond lens group moving ring to be rotatable relative to said secondlens group moving ring, and includes a plurality of outer cam groovesformed on an outer peripheral surface of said cam ring, and a pluralityof inner cam grooves formed on an inner peripheral surface of said camring; a plurality of inward cam followers which project radially inwardsfrom said first lens group moving ring to be engaged in said pluralityof outer cam grooves, respectively; and a plurality of outward camfollowers which project radially outwards from said second lens groupmoving ring to be engaged in said plurality of inner cam grooves,respectively.
 6. The retracting mechanism according to claim 1, whereinsaid biasing device comprises a compression coil spring.
 7. Theretracting mechanism according to claim 5, positions of said first lensgroup moving ring and said second lens group moving ring in said opticalaxis direction are adjusted by rotating said cam ring to make saidsecond follower introducing groove and said follower introducingthrough-slot aligned in said optical axis direction so that said secondfollower introducing grooves, said follower introducing through-slotsand said first follower introducing grooves form an L-shaped followerintroducing channel, through which said linear moving key is insertedinto said hanging groove, when said third lens group moving ring isinstalled in said zoom lens barrel during assembly.